The prior aft of laser construction and operation has demonstrated that the coupling between the counter-propagating waves in laser amplifiers and the accompanying effects of spatial hole burning and cross saturation effects are causes for a degradation of lasers' oscillation quality, and a reduction in its power and coherence. These effects can best be observed in the following two ways: (a) The reflection from the induced saturable gain grating due to the interference pattern (the Bragg condition is automatically fulfilled) are found to be destructively interfering (phase difference of 180.degree.) with the corresponding copropagating waves; thus providing a negative feedback, which in turn tends to eliminate this grating by a reduction of the coherence. (b) Alternatively, it can be verified that in most of the interesting cases, a uniform distribution of light intensity in saturable gain media, (corresponding to mutually incoherent counter-propagating beams) provides more amplification, compared to the nonuniform distribution of gain (as in a standing wave pattern) with the same intensity average, for the waves that create the pattern. In the high intensity sections, the saturation decreases the gain, while in the unsaturated places the gain is ineffective due to low or zero intensity. Therefore, more gain can be achieved for less coherent waves, as interference is reduced.
Moreover, this interference has an opposite effect on a third wave with a different frequency. Then, nonuniform periodic gain distribution due to the interference of the first pair provides more amplification for the signal compared to the uniform case, since the overlap between the intensity and the gain patterns (gratings) is higher. In the grating picture this corresponds to the situation where the Bragg condition is not met and the reflections are eliminated. This is a reason for the tendency to multi-longitudinal mode oscillation of lasers.
For many purposes, such as fiber optic communication, sensors, image processing, interferometry, and measurements, it would be desirable to improve the quality of laser oscillations and obtain single frequency operation.